Representative examples of the energy storage device may include a secondary cell battery and an electrochemical capacitor. The electrochemical capacitor is generally referred to as an electric double layer capacitor, a supercapacitor, or an ultracapacitor, and serves as an energy storage device to store and supply electric energy using capacitor behavior induced by an electrochemical reaction between an electrode and an electrolyte. Since there is almost no deterioration caused by repeating charging/discharging operations, these capacitors do not need repair work, and have high efficiency and semi-permanent lifespan with rapid charge and discharge capabilities, thereby being of great interest as an energy storage device capable of replacing or being used together with the secondary battery. Accordingly, the supercapacitor among these capacitors is expected to be employed in a variety of applications, for example, an IC back-up type device for a variety of electrical and electronic equipment, a pulse power supply for portable mobile communication devices, automobiles (electric, hybrid, fuel cell), toys, solar energy storage devices, and the like. A carbon nanotube or graphene is preferably used as an electrode material for a supercapacitor due to physicochemical properties thereof.
Conventionally, development of an energy storage device using nanomaterials still involves various limitations. More particular, even when a nanoscale material is synthesized or prepared, the reaction occurs only on the surface of the synthesized material to hence lack ability to reach an inherent theoretical capacity of the material. Accordingly, studies on decreasing a size of the energy storage material and dispersing the same have attracted great interest in the art, however, the range of the size is substantially restricted to several nanometers, which in turn, results in limitation in energy storage capacity and output characteristics. In order to reach the inherent theoretical capacity of the energy storage material, a technical solution of forming the material in an atomic state rather than in a nanoscale size may be proposed. Although a bottom-up method is generally used to prepare particles, these particles often agglomerate together to hence produce nanoparticles only in the form of an aggregate of hundreds of particles.
In consideration of this circumstance, the present inventor has found that the above problem in the energy storage device based on the scale of atomic units can be experimentally overcome by synthesizing metal oxide crystals on the scale of atomic units on the surface of graphene through lithiation for the first time in the world, and has completed the present invention. This result was verified and supported from simulations involving many particles such that lithium ions react with oxygen to form a Ni:Li2O core-shell structure. Also, in order to obtain a driving force for rescaling atomic unit scale nanocrystals, positive electrode surface charges may be applied. Further, X-ray photoelectron spectroscopy and in-situ optical spectrum analysis may be used to demonstrate that rescaled divalent Ni ions are reversely changed to a zero-valent state in a lithium intercalation/deintercalation cycle. Moreover, collecting particles rescaled at the positive electrode of an asymmetric full cell may enable full extraction of electrostatic capacitance (i.e. electric capacity) at a negative electrode which is an electrode opposite to the positive electrode. Further, the stored capacitance may retain a high current density while exhibiting long-life characteristics. Consequently, a novel capacitor with a high energy density, high capacitance and long-life characteristics may be anticipated to be useable as a next-generation energy storage device with improvements.
As a prior art relating to the supercapacitor, there are many documents including Korean Patent Laid-Open Publication No. 10-2013-0028423 (entitled “electrode for supercapacitor using graphene/metal oxide nanocomposite”), which relate to a development of the electric double-layer capacitors using activated carbon, pseudo-capacitors using metal oxide and conductive polymer, are the like. In this regard, these documents have been introduced to have an electrostatic capacitance of about 1 mF to 10,000 F. Meanwhile, the invention of the above laid-opened patent discloses a technique of using high conductivity and low resistance characteristics of graphene to overcome the low resistance of metal oxide and achieve effects of imparting high electrostatic capacitance. Consequently, an energy storage power source in a new concept, that is, a high energy density type next-generation supercapacitor could be attained. Korean Patent Registration No. 10-1199004 (entitled “a nano-porous electrode for supercapacitor and a method for preparing the same”) discloses a technique of mainly using metal oxide or conductive polymer as an electrode material for the supercapacitor. Among these, transition metal oxide-based raw materials are most greatly attracting public attention as such an electrode material for the supercapacitor nowadays. In particular, ruthenium oxide exhibits quite high specific condensing capacity, longer operation time, high electrical conductivity, and excellent high charge/discharge rate characteristics, and the like, and therefore, many studies have proceeded. On the other hand, the invention of the above registered patent discloses a technique of using electro-plating which involves hydrogen generation to form pores on the surface or inside of the electrode, which in turn increases a specific surface area and improves charge and discharge capacities, energy density and/or output density of the capacitor. Accordingly, as shown in FIG. 10, a ruthenium-copper porous metal structure had a specific condensing capacity of 1100 F/g, and as a result of charging/discharging over 3,000 cycles, it can be seen that this structure could retain a specific condensing capacity of about 750 to 800 F/g.
As described above, the conventional art still involves problems such as reduced charge and discharge capacities of a supercapacitor and a small number of cycles to hence entail difficulties in commercial utility thereof.